functions.php

<?php 

// the code below goes in your theme's functions.php file

function my_custom_pos_template_redirect() {
  
  $usd_store_id = 3365;
  $cad_store_id = 3366;
  $current_store_id = get_user_option('woocommerce_pos_store');
  
  if($current_store_id == $usd_store_id) {
    wc_setcookie( 'wcumcs_user_currency_cookie', 'USD', time() + 7776000 );
  }
  
  if($current_store_id == $cad_store_id) {
    wc_setcookie( 'wcumcs_user_currency_cookie', 'CAD', time() + 7776000 );
  }
  
}

// woocommerce_pos_template_redirect runs just before the POS template is rendered
add_action( 'woocommerce_pos_template_redirect', 'my_custom_pos_template_redirect', 20 );